For many years conventional display devices have been designed to be viewed by multiple users simultaneously. The display properties of the display device are made such that viewers can see the same good image quality from different angles with respect to the display. This is effective in applications where many users require the same information from the display—such as, for example, displays of departure information at airports and railway stations. However, there are many applications where it would be desirable for individual users to be able to see different information from the same display. For example, in a motor car the driver may wish to view satellite navigation data while a passenger may wish to view a film. These conflicting needs could be satisfied by providing two separate display devices, but this would take up extra space and would increase the cost. Furthermore, if two separate displays were used in this example it would be possible for the driver to see the passenger's display if the driver moved his or her head, which would be distracting for the driver. As a further example, each player in a computer game for two or more players may wish to view the game from his or her own perspective. This is currently done by each player viewing the game on a separate display screen so that each player sees their own unique perspective on individual screens. However, providing a separate display screen for each player takes up a lot of space and is costly, and is not practical for portable games. To solve these problems, multiple-view directional displays have been developed. One application of a multiple-view directional display is as a ‘dual-view display’, which can simultaneously display two or more different images, with each image being visible only in a specific direction—so an observer viewing the display device from one direction will see one image whereas an observer viewing the display device from another, different direction will see a different image. A display that can show different images to two or more users provides a considerable saving in space and cost compared with use of two or more separate displays.
Examples of possible applications of multiple-view directional display devices have been given above, but there are many other applications. For example, they may be used in aeroplanes where each passenger is provided with their own individual in-flight entertainment programmes. Currently each passenger is provided with an individual display device, typically in the back of the seat in the row in front. Using a multiple view directional display could provide considerable savings in cost, space and weight since it would be possible for one display to serve two or more passengers while still allowing each passenger to select their own choice of film.
A further advantage of a multiple-view directional display is the ability to preclude the users from seeing each other's views. This is desirable in applications requiring security such as banking or sales transactions, for example using an automatic teller machine (ATM), as well as in the above example of computer games.
A further application of a multiple view directional display is in producing a three-dimensional display. In normal vision, the two eyes of a human perceive views of the world from different perspectives, owing to their different location within the head. These two perspectives are then used by the brain to assess the distance to the various objects in a scene. In order to build a display which will effectively display a three dimensional image, it is necessary to re-create this situation and supply a so-called “stereoscopic pair” of images, one image to each eye of the observer.
Three dimensional displays are classified into two types depending on the method used to supply the different views to the eyes. A stereoscopic display typically displays both images of a stereoscopic image pair over a wide viewing area. Each of the views is encoded, for instance by colour, polarisation state, or time of display. The user is required to wear a filter system of glasses that separate the views and let each eye see only the view that is intended for it.
An autostereoscopic display displays a right-eye view and a left-eye view in different directions, so that each view is visible only from respective defined regions of space. The region of space in which an image is visible across the whole of the display active area is termed a “viewing window”. If the observer is situated such that their left eye is in the viewing window for the left eye view of a stereoscopic pair and their right eye is in the viewing window for the right-eye image of the pair, then a correct view will be seen by each eye of the observer and a three-dimensional image will be perceived. An autostereoscopic display requires no viewing aids to be worn by the observer.
An autostereoscopic display is similar in principle to a dual-view display. However, the two images displayed on an autostereoscopic display are the left-eye and right-eye images of a stereoscopic image pair, and so are not independent from one another. Furthermore, the two images are displayed so as to be visible to a single observer, with one image being visible to each eye of the observer.
For a flat panel autostereoscopic display, the formation of the viewing windows is typically due to a combination of the picture element (or “pixel”) structure of the image display unit of the autostereoscopic display and an optical element, generically termed a parallax optic. An example of a parallax optic is a parallax barrier, which is a screen with transmissive regions, often in the form of slits, separated by opaque regions. This screen can be set in front of or behind a spatial light modulator (SLM) having a two-dimensional array of picture elements to produce an autostereoscopic display.
FIG. 1 is a plan view of a conventional multiple view directional device, in this case an autostereoscopic display. The directional display 1 consists of a spatial light modulator (SLM) 4 that constitutes an image display device, and a parallax barrier 5. The image display SLM 4 of FIG. 1 is in the form of a liquid crystal display (LCD) device having an active matrix thin film transistor (TFT) substrate 6, a counter-substrate 7, and a liquid crystal layer 8 disposed between the substrate and the counter substrate. The SLM is provided with addressing electrodes (not shown) which define a plurality of independently-addressable picture elements, or “pixels”, and is also provided with alignment layers (not shown) for aligning the liquid crystal layer. The pixels of the image display SLM 4 are arranged in rows and columns with the columns extending into the plane of the paper in FIG. 1. A linear polariser 10 is provided over the outer surface of the substrate 7 nearest to an observer, and a viewing angle enhancement film 9 may optionally be placed between the polariser 10 and the substrate 7. Illumination is supplied from a backlight 11.
The parallax barrier 5 comprises a substrate 12 with a parallax barrier aperture array 13 formed on its surface adjacent the image display SLM 4. The aperture array comprises vertically extending (that is, extending into the plane of the paper in FIG. 1) transparent apertures 15 separated by opaque portions 14. A linear polariser 16 is formed on the surface of the parallax barrier substrate 12 facing the backlight 11. A further viewing angle enhancement film 9 may optionally be provided between the parallax barrier 5 and the image display SLM 4.
In the display 1 of FIG. 1, the parallax barrier 5 is also in the form of a liquid crystal display (LCD) device, in which a liquid crystal layer 18 is disposed between the substrate 12 and a counter substrate 17. The transparent apertures 15 and opaque portions 14 of the parallax barrier are defined in the liquid crystal layer 18 by suitably addressing the liquid crystal layer, and the parallax barrier LCD is provided with addressing electrodes (shown in FIG. 2(a)) which define the transparent apertures 15 and opaque portions 14 of the parallax barrier. The parallax barrier LCD is also provided with alignment layers (not shown) for aligning the liquid crystal layer 18.
FIG. 2(a) is a cross-section through the parallax barrier 5, and FIG. 2(b) is a plan view of the parallax barrier 5. As described above, the parallax barrier is in the form of a liquid crystal display (LCD) device having a substrate 12, a counter-substrate 17, and a liquid crystal layer 18 disposed between the substrate 12 and the counter substrate 17. The SLM is provided with addressing electrodes Ei (where i=1, 2, 3 . . . ) on the substrate 12. These electrodes Ei are shown in plan view in FIG. 2(b) and, as can be seen, they are generally stripe-shaped and extend parallel to but spaced from one another. A counter electrode 21 is provided on the counter substrate 17. Driving circuitry is provided for addressing the electrodes Ei, and this is shown schematically as 22 in FIG. 2(b). Other components such as alignment films are omitted from FIGS. 2(a) and 2(b) for clarity.
In a case where the liquid crystal device is “normally white”, a parallax barrier may be defined by addressing the electrodes Ei such as to make the corresponding regions of the parallax barrier SLM opaque to form the opaque regions 14 of the parallax barrier. Thus, each electrode defines one opaque region 14 of the parallax barrier. The regions of the parallax barrier SLM corresponding to the gaps between adjacent electrodes remain maximally transmissive to form the transmissive regions 15 of the parallax barrier. Conversely, if the liquid crystal device is “normally black”, a parallax barrier is defined by addressing the electrodes Ei such as to make the corresponding regions of the parallax barrier SLM maximally transmissive to form the transmissive regions 15 of the parallax barrier; the regions of the parallax barrier SLM corresponding to the gaps between adjacent electrodes remain opaque and form the opaque regions 14 of the parallax barrier.
In use, two images are displayed on the liquid crystal layer 8 of the image display SLM 4, for example with the two images being interlaced on the columns of pixel. The parallax barrier 5 selectively blocks light so that light passing through a pixel of the liquid crystal layer 8 of the image display SLM 4 is travelling only in a restricted range of directions. The display device 1 thus forms a left-eye image and a right-eye image, and an observer who positions their head such that their left and right eyes are coincident with the viewing window 19 for the left eye image (or the “left viewing window”) and the viewing window 20 for the right eye image (or the “right viewing window”) respectively will see a three-dimensional image. The left and right viewing windows 19,20 are formed in a window plane at the desired viewing distance from the display.
While the display of FIG. 1 is effective at providing an autostereoscopic 3-D display, an observer will see a 3-D display only if their left and right eyes are aligned with the left and right viewing windows 19,20 respectively. If an observer moves their head such that their left and right eyes are no longer aligned with the left and right viewing windows 19,20, they will cease to see a 3-D image. The lateral width w of the left and right viewing windows 19,20 is typically of the order of 6 cm, so an observer can move their head by no more than this distance if they are to continue to see a 3-D image. The display is said to have low “viewing freedom”.
A dual view display is similar in concept to the display 1 of FIG. 1, except that the display displays a first image to an observer with their head positioned in one viewing window and displays a second image to another observer with their head positioned in a second viewing window. Again, an observer must keep their head positioned in the appropriate viewing window in order to see the intended image, and if the observer moves such that their head is no longer in the viewing window they will no longer see the intended image; the movement of the observer is therefore constrained and the display again has a low viewing freedom. While this may not be of consequence in, for example, a motor vehicle where the occupants have only limited freedom to move, it is a significant problem in some possible applications of a dual view display.
There has been considerable effort to increasing the viewing freedom of a multiple view display by eliminating the need for an observer to remain with their head positioned in a constrained region of space in order to see a 3-D image or the intended image of a dual view display. In general, the proposed solutions involve the two steps of (1) monitoring the position of the observer's head and (2) adjusting the positions of the viewing windows of the display in dependence on the determined position of the observer's head. This is known as “observer tracking”. As an example, European patent application No. 98302989.3 discloses a method of a method of analysing a scene of video footage and determining a user's position in that scene; the determined position of the user may then be used to adjust the position of the viewing windows of a display.
The position of the viewing windows 19, 20 of the display of FIG. 1 may be altered by moving the opaque regions 14 and transmissive regions 15 of the parallax barrier 5 laterally with respect to the image displayed on the liquid crystal layer 8 of the image display SLM 4. In a display in which the parallax barrier is a fixed parallax barrier—ie, the opaque regions 14 of the parallax barrier are permanently opaque and the transmissive regions 15 of the parallax barrier are permanently transmissive, this may be done by mechanically translating the entire parallax barrier 5 relative to the image display SLM 4. However, this introduces moving parts and so leads to wear and unreliability; the mechanism for translating the barrier also adds weight and bulk to the display. Attention has therefore been directed towards a reconfigurable parallax barrier, in which the opaque regions and the transmissive regions of the parallax barrier are not permanently defined, so that the parallax barrier may be reconfigured by changing the areas of the parallax barrier that are opaque and correspondingly changing the areas of the parallax barrier that are transmissive. The parallax barrier 5 shown in FIG. 1 is a reconfigurable parallax barrier; the transmissive regions 15 and opaque regions 14 are defined in the liquid crystal layer 18 of the parallax barrier 5, and the position of the transmissive regions 15 and opaque regions 14 may be changed by re-addressing the liquid crystal layer 18. Use of a reconfigurable parallax barrier allows the transmissive regions 15 and opaque regions 14 of the parallax barrier to be moved laterally relative to the image display SLM 4 so as to alter the lateral position of the viewing windows 19,20 without the need to translate the entire parallax barrier relative to the SLM 4.
To provide observer tracking, the display 1 is further provided with a position determining portion 61 for determining the position of the observer. The position determining means may be, for example, a camera 62 directed towards the intended position of an observer, and an analyser 63 for determining the position of an observer from an image obtained by the camera 62 (for example according to the method of European patent application No. 98302989.3).
The display 1 further has a controller 64 for controlling the parallax barrier 5. The controller controls the parallax barrier in accordance with the position of the observer as a determined by the analyser 63—the positions of the opaque regions and transmissive regions of the parallax barrier are controlled in dependence on the determined position of the observer such that the position of the left and right viewing windows 19,20 coincide with the instantaneous positions of the observer's eyes.
European patent application No. 97307571.6 describes a method of providing a reconfigurable parallax barrier based on LCD fringing fields. However, this can be difficult to achieve with many common LC modes.
U.S. Pat. No. 6,049,424 discloses a method of providing a reconfigurable parallax barrier; this method is illustrated in FIGS. 3(a) to 3(c).
The parallax barrier of U.S. Pat. No. 6,049,424 is similar to that of FIG. 2(a), in that it uses a liquid crystal SLM. However, the electrodes of the parallax barrier are narrower than the intended opaque regions 14 of the parallax barrier, and an opaque region is defined by addressing a number of adjacent electrodes. Moreover, each of the electrodes Ei is addressable independently of the others. In contrast, in the parallax barrier 5 of FIG. 2(a), each electrode Ei defines one opaque (or transmissive) region of the parallax barrier.
FIG. 3(a) is a plan view of the SLM of U.S. Pat. No. 6,049,424. It shows electrodes E1, E2, E3, E6, E7, E8, E11 etc. addressed so as to make the corresponding regions of the SLM opaque, while electrodes E4, E5, E9, E10 etc. are addressed so as to make the corresponding regions of the SLM transmissive. The regions of the SLM corresponding to electrodes E1, E2, E3 constitute one opaque region 14 of the parallax barrier, the regions of the SLM corresponding to electrodes E5, E6 constitute one transmissive slit 15 of the parallax barrier, the regions of the SLM corresponding to electrodes E6, E7, E8 constitute a second opaque region 14 of the parallax barrier, and so on.
The parallax barrier may be reconfigured by re-addressing the electrodes, and this is shown in FIGS. 3(b) and 3(c). In FIG. 3(b), electrodes E2, E3, E4, E7, E8, E9, E12 ect. are addressed so as to make the corresponding regions of the SLM opaque while electrodes E1, E5, E6, E10, E11 etc. are addressed so as to make the corresponding regions of the SLM transmissive, and in FIG. 3(c), electrodes E3, E4, E5, E8, E9, E10 etc. are addressed so as to make the corresponding regions of the SLM opaque while electrodes E1, E2, E6, E7, E11, E12 etc. are addressed so as to make the corresponding regions of the SLM transmissive. The effect of reconfiguring the parallax barrier is that the opaque regions and transmissive regions of the parallax barrier “move” laterally across the SLM. If such an SLM is used to provide the parallax barrier in a multiple view display, it is possible to change the positions of the viewing windows 19,20 so as to track a moving observer.
The method of U.S. Pat. No. 6,049,424 has the disadvantage that there is necessarily a gap 23 between adjacent electrodes; these gaps are typically 10 μm wide, and each gap 23 will lead to a corresponding region of the liquid crystal layer that is not addressed. The unaddressed regions will appear as narrow transmissive stripes within an opaque region 14 of the barrier and will degrade the 3-D (or dual view) performance of the display by allowing cross-talk (“cross talk” occurs where an image intended for viewing from the first [or second] viewing window 19 [20] is also visible from the second [or first] viewing window 20 [19]). Moreover, each electrode Ei must be individually addressed, and this requires expensive drive circuitry.